This invention relates to a sample chamber for an optical spectrometer. Particular embodiments of the invention disclosed are sample chambers for an infrared Fourier-transform spectrometer which can maintain samples and infrared beams in a vacuum.
Optical spectrometers measure the fraction of energy absorbed by a sample of material from a beam of optical radiation as a function of the frequency of the radiation. As used herein the term "optical beam" refers to a beam of electromagnetic radiation whose frequency spectrum lies in the infrared, visible and ultraviolet ranges. The energy absorbed by samples may be studied in two different ways by measuring either the transmittance or reflectance of the sample. In a transmission experiment the optical beam is passed through the sample and energy absorbed from the beam is detected by comparing the intensity of the beam impinging upon the sample to the intensity of the beam passing out of the sample. In a reflection experiment the energy absorbed by the sample is determined by comparing the intensity of a beam impinging upon a surface of the sample to the intensity of the beam reflected from the surface. Reflectance measurements are usually made on solid samples, while transmittance measurements are commonly made on both solids and fluids.
In designing sample chambers for use in making these two types of measurements, attention must be paid to the orientation of the sample relative to the beam. Ordinarily a sample, be it a solid or fluid, is prepared or confined so that the surface on which the beam impinges is flat. The angle between the direction of propagation of the optical beam and the perpendicular to the surface on which the beam impinges is called the angle of incidence. For transmission measurements the sample is ordinarily oriented so that the angle of incidence is zero degrees. If the beam impinges upon a sample with an angle greater than zero degrees, then in passing through the sample the axis of the beam will be displaced. The amount of displacement depends upon the thickness of the sample, the index of refraction of the sample, and the angle of incidence. Such a displacement of the beam is undesirable because the optics of the spectrometer must be realigned to compensate for the displacement. When the angle of incidence of the beam is zero degrees, the beam passing from the sample is not displaced. For this reason sample chambers for use in transmission studies are designed so that samples are oriented to make the angle of incidence zero degrees. For reflectance measurements, however, the angle of incidence cannot be zero degrees. If it were, the reflected beam would coincide with the incident beam and it would be difficult to separate the two. Sample chambers for use in reflectance measurements are for this reason designed with the angle of incidence of the beam greater than zero degrees.
It is desirable to use the same spectrometer for both transmission and reflection measurements. Moreover it is useful to be able to make the changeover between these two modes of operation quickly and easily. Making this changeover, however, can be troublesome with certain spectrometers; particularly, those which operate in a vacuum or a special atmosphere. Changing the mode of operation of the spectrometer requires changing the path of the beam and often involves changing the orientation of the sample. These two procedures often involve opening the sample chamber, which in the case of vacuum spectrometers is a time-consuming and troublesome procedure because the vacuum seal must be broken, the sample chamber vented to the atmosphere, the changeover made, then the chamber must be resealed and reevacuated.